In order to maintain signal transmission quality of a transmission line in a higher frequency for a higher data transmission rate, whole characteristic impedance matching should be taken into consideration in a circuit design phase. Therefore, the characteristic impedance matching is even more critical for high-frequency circuit throughout a transmission line. For assuring of satisfactory high-frequency signal transmission accuracy, the characteristic impedance throughout the transmission path needs to be made consistent.
For improving the characteristic impedance matching effect, a variety of factors including width of traces, clearance between traces, plating hole structures of a multilayer printed circuit board, e.g. non-through holes or through holes, and conductor structures in the holes need to be precisely considered and well designed. For example, the characteristics of each plating hole, including pad size, pad shape, hole depth, and hole type, e.g. through hole, via, blind hole or buried hole, location of stub portion without signal transmission, and conductive structure inside the multilayer board, would have effects on signal transmission quality more or less.
In conventional designs, blind holes, buried holes or back-drilled holes are adopted to ameliorate signal reflection from the non-transmission stub portion. Please refer to FIG. 1A. For making traces of a circuit board from the first layer L1 to the fourth layer L4 by way of through holes, it is generally necessary to remove the stub portions ST of the circuit board disposed between the fourth layer L4 and the eighth layer L8. It is understood that the removal of the stub portions ST by back drilling requires precise control. Over drilling, as exemplified in FIG. 1B, would result in poor connection between traces, and insufficient drilling, as exemplified in FIG. 1C, would cause local stub residual. Both the situations would have adverse effects on the resulting high-frequency characteristic impedance, and furthermore, the yield rate of products as well as costs.